Biomolecule Protective and Photocatalytic Potential of Cellulose Supported MoS 2 /GO Nanocomposite

In the current study


Introduction
Environmental pollution is increasing day by day due to diferent industrial activities and urbanization, and hydrosphere has become badly polluted [1]. Tese pollutants are harmful for human beings and aquatic life as well [2]. According to an estimate, 50% of the world's population has been facing the scarcity of water and 900 million people have been deprived of the fresh water resources. Due to the polluted water, about 6 million people and 1.8 million children die every year due to diferent waterborne diseases. Pakistan is facing the same issue of water pollution due to poor sanitation system, industrialization, and urbanization [3]. Te most common sources of water pollution are industries such as textile, paper and pulp, and leather. A huge amount of industrial efuent is being discharged by industries to water streams. [4]. Industrial efuent usually contains toxic chemicals especially dyes that can cause a lot of damage to humans and aquatic animals. Tere are so many techniques and methodologies which are being applied in diferent countries of the world for the removal of dyes from the aqueous medium [5], and nanotechnology is one of the best among all of those. Terefore, nanomaterials including metal nanoparticles and graphene oxide-based nanocomposites having small size and large surface area can be used for wastewater treatment [6,7]. Metal nanoparticles Fe, Ag, metal oxides (TiO 2 , V 2 O 5 ) nanoparticles, and nanocomposites have been synthesized for the catalytic removal of dyes from wastewater [8,9].
Research studies have reported the antioxidant role of molybdenum nanoparticles along with photocatalytic applications for the degradation of ketamine [10]. Presence of free radicals and reactive oxygen/nitrogen species can damage biomolecules in the human body and instigate many diseases. Deterioration in lipid-containing food products or any other biomaterial may appear due to presence of free radicals or reactive species [11]. Free radical scavenging compounds are very helpful as they stop the oxidation process and protect biomolecules from damage. Nowadays, scientists have been using multiple compounds (synthetic, natural, and nanoparticles) as antioxidants to overcome this problem [12,13].
Tese days, the synthesis of nanoparticles by green/ simple, cost-efective methods has gained attention, and now, there is a big need to use the fast and cost-efective methods for the synthesis of nanomaterials [14,15]. Keeping in view the importance of nanocomposites as catalytic and radical scavenging materials, cellulose-supported MoS 2 @ GO nanocomposite has been synthesized. Synthesis was planned to be performed using a simple and fast hydrothermal method by avoiding excessive use of chemicals. Characterization was conducted by UV-visible, FTIR, SEM, and XRD techniques. Te nanocomposite was then tested for the degradation of 4-nitrophenol which is an organic pollutant that is usually discharged by pharmaceutical industries and two dyes that are found in textile industry efuents. In addition, photocatalytic potential of the nanocomposite was determined for the degradation of selected dyes. Biomolecule protective efciency was tested by measuring radical scavenging potential of nanocomposite employing DPPH • and ABTS •+ assays. Tis research work will motivate the researchers working on synthesis and applications of nanomaterials to prefer the methods that need relatively less eforts, use of chemicals, and resources.

Chemicals and Reagents.
Methylene blue (99.99%) was purchased for the Fisher Scientifc UK; methyl orange (99.9%) was purchased from the Sigma Aldrich from Germany. Cellulose, graphite, ammonium molybdenite, hydrogen peroxide, sodium nitrate, hydrochloric acid, ethanol, 4-nitrophenol, and nitric acid were also purchased from the Sigma Aldrich from Germany. All the reagents were of analytical grade, and no further purifcation was needed for laboratory use.

Synthesis of Cellulose
Nanofber. Cellulose nanofber was prepared by socking soft wood pulp sheet in water on a hot plate for 12 hours at 25°C (Figure 1(a)). Ten, swollen pulp was disintegrated for 10-20 min using a blender. Te resulting slurry was then added to a conical fask, Zirconia balls were added, and milling was conducted at 100°C for 4 h. Nanofbers of cellulose were obtained and used for the synthesis of nanocomposite [16].

Synthesis of MoS 2 Nanoparticles (Centrifuging Method).
Nanoparticles (NPs) of MoS 2 were prepared (Figure 1(b)) using ammonium molybdenite as a precursor. In 50 mL of acetic acid, 1.3 g of ammonium molybdenite was added followed by adding 100 mL of distilled water. It was placed on a hot plate with continuous stirring at 90°C for 30 min, and then ammonia water was added to it and stirred for further 4 h at 50°C. In the resulting mixture, 2 g of urea was added and stirring was done for 5 min to form the gel type material. Viscous solution (40 mL) was transferred into the autoclave, and temperature was kept at 160°C for 10 h. After removing the autoclaved solution from the oven, it was cooled for 5 h at room temperature. Centrifugation was performed at 40 rpm for 30 min, and precipitates were collected and washed with distilled water and ethanol to remove impurities. Te fnal product was obtained after drying in the oven [16].

Synthesis of GO Nanoparticles (Modifed Hammer
Method). Graphene oxide was synthesized employing the modifed Hammer's method (Figure 1(c)). Graphite (5 g) and NaNO 3 (2.5 g) were mixed in a beaker (mixture A). In another beaker, 105 mL of sulphuric acid and 12 mL of H 3 PO 4 were mixed (mixture B). Mixture A was poured into mixture B and after mixing placed on an ice bath for 10 min. In the resulting mixture, 15 g of KMnO 4 was added and temperature was maintained below 5°C for 60 min. As a result, suspension was obtained. Te solution was then removed from ice bath and placed on stirrer 98°C with continuous addition of water. After sometime, 15 mL of H 2 O 2 was added to the reaction mixture and centrifuged at 4000 rpm for 30 min. Te fnal product was washed with distilled water and 20% HCl for three times to get the GO nanoparticles [17,18]. (Hydrothermal Method). Nanocomposite cellulose/MoS 2 /GO was synthesized (Figure 1(d)) by a two-step process. In the frst step, 2 grams of cellulose nanoparticles and 1 gram of MoS 2 in distilled water were used followed by stirring for 60 min in a hot plate. A viscous solution of cellulose and MoS 2 appeared. It was shifted to the autoclave and heated in the oven at 200°C for 4 h. After 4 h, the autoclave was cooled to room temperature. Centrifugation was performed, and precipitates were separated at 35000 rpm for 20 min. Precipitates were washed with distilled water/ethanol for three times and dried at room temperature for three days. In the second step, cellulose/MoS 2 was dissolved in distilled water and 0.5 g of GO was mixed. Te resulting mixture was heated 50°C for 2 h followed by sonication for 10 min. After sonication, 100 mL of material was shifted to the autoclave and  transferred to the oven at 300°C for 4 h. Te autoclave was then cooled at 25°C for 4 h, and centrifugation was conducted for 30 min. Te precipitates were collected and washed with distilled water and ethanol for three times to remove all types of impurities. Te fnal product was dried for 7 days at room temperature to get the grey color cellulose/MoS 2 /GO nanocomposite.

Characterization.
Te characterization of the synthesized nanoparticles and nanocomposite was conducted with the help of UV-visible (CECIL 7400-ce Aquarius Cambridge, UK) and FTIR spectrophotometer (Bruker alpha (II), UK). Te scanning electron microscopic (SEM) analysis was performed using NOVA FE-SEM 450. Te X-ray difraction (XRD) analysis of the nanocomposite was also performed using the Bruker difractometer (Coventry, UK). Characterization confrmed the formation of nanoparticle and nanocomposite.

Biomolecule Protective Potential.
Biomolecule protective potential of the nanocomposite was evaluated in terms of free radical (DPPH • and ABTS •+ ) scavenging activities. Antioxidant activities of nanocomposite cellulose/MoS 2 @ GO were determined using DPPH • and ABTS •+ assays. One milligram of nanocatalyst was added separately to the test tube solution containing 1 mL of DPPH • (0.01 mM) and ABTS •+ (0.03 mM) followed by addition of 5 mL of methanol and 4 mL of water. Te scavenging of free radicals was determined by recording their spectra using a UVvisible spectrophotometer after regular intervals. Percent- Here, A d is the absorbance of pure DPPH • and ABTS •+ solutions, and A s is the absorbance of the sample [13,19].

Photocatalytic Potential of Cellulose/MoS 2 /GO.
Photocatalytic potential of the nanocomposite was determined in terms of its dye degradation potential under sunlight by following a method reported earlier [20]. Solutions of dyes such as methylene blue (1 mM) (λ max value of 667 nm) and methyl orange (1 mM) (λ max of 467 nm) were prepared. Catalytic degradation of methylene blue was carried out in direct sunlight by taking 0.01 mM solution of methylene blue in 5 diferent test tubes having 1, 3, 5, 7, and 10 milligram of catalyst being added to them, and the tubes were placed in direct sunlight. Degradation was observed using a UV-visible spectrophotometer after regular intervals.
In the same way, catalytic degradation of methyl orange was also performed and results were recorded [21]. Another organic pollutant, i.e., 4-nitrophenol, commonly found in pharmaceutical efuent, was degraded using cellulose/MoS 2 / GO. Solution of 4-nitrophenol solution was prepared by adding its 0.0139 gram in 100 mL of water to get 1 mM solution having λ max at 400 nm. Te degradation of 4nitrophenol was also performed under sunlight in 5 different test tubes with 5 mL 1 mM solution, and degradation was recorded using a UV-visible spectrophotometer [22].

UV-Visible Analysis.
Analysis of all the products including cellulose nanofber, GO, MoS 2 , and cellulose/MoS 2 / GO nanocomposite was performed by using a UV-visible spectrophotometer. In UV-visible analysis, the observed value for cellulose appeared at 358 nm, comparable with the already reported value at 360 nm [23] (Figure 2(a) (i)). Te UV-visible spectra for MoS 2 was also recorded, and a peak was observed at 345 nm which was close to the reported value 340 nm [24] that confrmed the formation of MoS 2 nanoparticles (Figure 2(a) (ii)). In UV-visible analysis, GO showed a peak at 355 nm which is in close resemblance with an already reported value at 350 nm as shown in Figure 2(a) (iii). Nanocomposite, i.e., cellulose/MoS 2 /GO, was also analyzed using a UV-visible spectrophotometer, and spectra were obtained having a peak at 348 nm which does not resemble with any spectra of the individual component (cellulose, MoS 2 , and GO). It confrms the association of all the components involved in nanocomposites, as it lies in between the values of nanoparticles and the nanocomposite (Figure 2(a) (iv)). Te plot between (ahv) 1/2 vs. energy (eV) presenting the band gap of nanocomposites, MoS 2 NPs and GO. Te GO has the highest energy band gap (3.90 eV), while MoS2/GO have intermediate and it was found having the lowest energy band gap. It reveals that nanocomposite can easily provide electrons necessary to be available for photocatalysis, and the same results have been reported in previous studies [25]. Correspondingly, nanocomposites with GO showed a similar band gap energy of 3.52 eV as reported earlier [26].

Fourier Transform Infrared Analysis.
Te FTIR analysis of cellulose nanofber, GO, MoS 2 , and cellulose/MoS 2 /GO nanocomposite was performed. In Figures 3 (a)-(d), spectra for cellulose nanofber, MoS 2 , GO nanoparticles, and cellulose/MoS 2 /GO nanocomposite have been presented. FTIR spectra revealed diferent identities of nanoparticles as well as nanocomposite. In Figure 3 (a), (cellulose nanofber) peak at 800 cm −1 shows the presence of an aromatic compound [27], a broad absorption band at 3,333-3,400 cm −1 , which corresponds to hydroxyl group (-O-H stretching) vibrations in cellulose [28]. Another peak observed at 1,057 cm −1 may be due to skeletal vibration in -C-O-C and β-glycosidic at 897 cm −1 [29]. Formation of MoS 2 nanoparticles was confrmed by the FTIR analysis (Figure 3 (b)) that shows diferent peaks as representative of MoS 2 nanoparticles. A stretching peak observed around 610 cm −1 can be attributed to the stretching of Mo-S bond [30,31]. Te spectrum recorded for GO (Figure 3 (c)) was found containing broad band around 3,400 cm −1 that is a strong indication of OH stretching. Second, a peak around 1,600 cm −1 indicates the stretching of C�C, and peaks at 1,800 cm −1 , 1,200 cm −1 , and 1,020 cm −1 may be due to the presence of C�O, C-OH, and C-O, respectively [32,33].
In Figure 3 (d), the FTIR spectrum recorded for nanocomposite (cellulose/MoS 2 /GO) has been presented. All the characteristic peaks of cellulose nanofber, MoS 2 nanoparticles, and GO were obtained in the spectrum of the nanocomposite. It confrms the association among all the components of the nanocomposite. Te peak for skeletal vibration in -C-O-C was observed at 1,057 cm −1 , and a band appearing at 590 cm −1 indicated the presence of Mo-S and S-S linkage. In addition, representative peaks of GO can also be observed in the spectrum. All the evidence obtained after FTIR spectra confrmed the formation of not only precursors but the nanocomposite as well. Figure 4) (Table 1). Te curve at 22.45°shows the amorphous presence of cellulose, and at 26.34°, (110) represents GO with orthorhombic crystalline nature. XRD studies confrmed the formation of nanocomposite [34]. Te nanocomposite crystalline phase was hexagonal with PDF#73-1508. Te crystallite size was 6.447 nm using Debye Scherrer equation D (nm) � kλ/β cosθ, where D (nm) represents crystallite size, k denotes constant, β is full-width half maximum, and θ is the angle. Te details of hkl and interplanar distance (Å) with 2θ are mentioned in Table 1. Likewise, results were also reported by some researchers recently [35,36].

SEM Analysis.
Te size and morphology of nanocomposite was evaluated by scanning electron microscopy. Te SEM images ( Figure 5) were taken at diferent resolutions. Te diameter of synthesized nanocomposite was noted up to 50-80 nanometres having a heterogeneous surface. Such surface of the synthesized nanocomposite may be suitable if it is used as a photocatalyst for degradation of dyes [37]. Te nanoparticles of MoS 2 stabilized by cellulose can be clearly seen at the surface of GO (Figure 5(a)), and the GO nanosheet's surface without cellulose stabilized nanoparticle loading is shown in Figure 5  nanocomposite was used as a catalyst to carry out the degradation process in a smooth and accelerated manner. A pollutant 4-nitrophenol was subjected to degradation using cellulose/MoS 2 /GO nanocomposite under sunlight, and results have been shown in the Figure 6. Results show that the catalyst has signifcantly contributed towards degradation of 4-nitrophenol. In the absence of the catalyst, degradation was not observed, and after adding the nanocomposite, degradation of 4-nitrophenol up to 75% was achieved just in 12 min. Graphene oxide in combination with MoS 2 has been reported for the degradation of 4nitrophenol [38,39].
Many researchers have reported the use of MoS 2 -based nanomaterials for the degradation of dyes from diferent effuents [40]. In the same way, degradation of methylene blue was achieved using the nanocomposite under sunlight directly. Photocatalytic degradation was observed at λ max 667 nm using a UV-visible spectrophotometer. Te degradation was started after adding the nanocomposite, and maximum degradation up to 85% was achieved in 10 min (Figure 7). Results of the current study may be considered better in comparison with already reported for the degradation of methylene blue dye using molybdenum-based nanomaterials [41].
Te role of molybdenum-based nanomaterials for the removal of organic contaminants has already been reported in many studies [42]. Te degradation of methyl orange was also performed under sunlight directly, and the process was observed using a UV-visible spectrophotometer at λ max of 467 nm. Degradation of the dye takes place after adding the nanocomposite to the dye solution. Degradation up to 70% was achieved (Figure 8) that is comparable with the degradation potential of molybdenum composite, already reported for methyl orange [43].
3.6. Biomolecule Protective Potential. Reports revealed that MoS 2 nanomaterials can be used for the removal of reactive oxygen species that are responsible for oxidative stress and biomolecule damages [44]. Biomolecule protective potential of the nanocomposite has been determined by evaluating radical (DPPH • and ABTS •+ ) scavenging potential. Te DPPH • assay was performed, and the nanocomposite was allowed to react with DPPH free radicals. Decrease in concentration of free radicals was observed using a UVvisible spectrophotometer at 530 nm after regular interval of times ( Figure 9). Decrease in absorbance of the solution indicated the scavenging of free radicals, and the maximum amount of radicals (45%) was neutralized by the nanocomposite in 24 min. Te remaining amount of the free radicals can be removed using a high concentration of the nanocomposite. MoS 2 -based nanocomposite synthesized in the current study exhibited improved DPPH radical scavenging potential as compared to that reported earlier [45].
With the help of cellulose/MOS 2 / GO, the radical scavenging of ABTS •+ was determined. Radical scavenging was observed with the help of a UV-visible spectrophotometer at 651 nm. Te nanocomposite exhibited maximum radical scavenging potential in 49 min, and 45% of the radicals were neutralized (Figure 10).
Comparative potential of the nanocomposite for the removal of selected dyes and free radicals has been presented in Table 1. It is clear that nanocomposite is potent enough for the catalytic degradation methylene blue as compared to other two dyes. However, the efciency of the nanocomposite for the neutralizing free radicals was found to be same. Te efciency of cellulose/MoS 2 /GO for the removal of dyes/free radicals has been compared with the other molybdenum-based nanoparticles/nanocomposite. Removal efciency was found to be lesser as compared to the previously reported data. It may be due to the cellulosic material Table 2 covering around MoS 2 and GO.

Conclusion
Synthesis of nanomaterials has been focused by many researchers due to their vast spectrum of applications. Most of the scientists are focusing to synthesize nanomaterials by simple methods with lesser involvement of toxic chemicals. Tis target has been achieved by synthesizing cellulose/ MoS 2 /GO nanocomposite material by the simple, ecofriendly, and cost-efective hydrothermal method without compromising on activity potential. Tis work will provide a path for the researchers to adopt simple methodologies for the fabrication of nanomaterials of their interest. Te synthesized nanocomposite was found having size ranging from 50 to 80 nm and heterogeneous structure. It was found active against ABTS •+ and DPPH • , providing an evidence for its biomolecule protective nature. In addition, degradation of 4nitrophenol, methylene blue, and methyl orange was catalyzed by cellulose/MoS 2 /GO, and the results confrmed photocatalytic potential of the nanocomposite. Authors strongly recommend the hydrothermal synthesis method for the fabrication of nanomaterials. Nanocomposite cellulose/ MoS2/GO synthesized in this study can be used for the protection of biomolecules from free radicals. It can also be used as a nanocatalyst for the removal of 4-nitrophenol found in pharmaceutical efuent and dyes from textile industry wastewater.

Data Availability
No data were used to support the fndings of this study.

Conflicts of Interest
Te authors declare that they have no conficts of interest.